Space Pollution

Photo by: Stéphane Bidouze

In the most general sense, the term space pollution includes both the
natural micrometeoroid and man-made orbital debris components of the space
environment; however, as "pollution" is generally considered
to indicate a despoiling of the natural environment, space pollution here
refers to only man-made orbital debris. Orbital debris poses a threat to
both manned and unmanned spacecraft as well as the earth's
inhabitants.

Environmental and Health Impacts

The effects of debris on other spacecraft range from surface abrasion due
to repeated small-particle impact to a catastrophic fragmentation due to a
collision with a large object. The relative velocities of orbital objects
(10 kilometers per second [km/s] on average, but ranging from meters per
second up to 15.5 km/s) allow even very small objects—such as a
paint flake—to damage spacecraft components and surfaces. For
example, a 3-millimeter (mm) aluminum particle traveling at 10 km/s is
equivalent in energy to a bowling ball traveling at 60 miles per hour (or
27 m/s). In this case, all the energy

would be distributed in an area of the same size as the particle, causing
cratering or penetration, depending on the thickness and material
properties of the surface being impacted. There has been one accidental
collision between cataloged objects to date, but surfaces returned from
space and examined in the laboratory confirm a regular bombardment by
small particles. Space Shuttle vehicle components, including windows, are
regularly replaced due to such damage acquired while in orbit. Debris also
poses a hazard to the surface of the Earth. High-melting-point materials
such as titanium, steel, ceramics, or large or densely constructed objects
can survive atmospheric reentry to strike the earth's surface.
Although there have been no recorded fatalities or severe injuries due to
debris, reentering objects are regularly observed and occasionally found.

Debris is typically divided into three size ranges, based on the damage it
may cause: less than 1 centimeter (cm), 1 to 10 cm, and larger than 10 cm.
Objects less than 1 cm may be shielded against, but they still have the
potential to damage most satellites. Debris in the 1 to 10 cm range is not
shielded against, cannot easily be observed, and could destroy a
satellite. Finally, collisions with objects larger than 10 cm can break up
a satellite. Of these size ranges, only objects 10 cm and larger are
regularly tracked and cataloged by surveillance networks in the United
States and the former Soviet Union. The other populations are estimated
statistically through the analysis of returned surfaces (sizes less than 1
mm) or special measurement campaigns with sensitive radars (sizes larger
than 3 mm). Estimates for the populations are approximately 30 million
debris between 1 mm and 1 cm, over 100,000 debris between 1 and 10 cm, and
8,800 objects larger than 10 cm.

The number, nature, and location of objects greater than 10 cm in size are
provided in the fragmentation debris table and in the image of space
debris around Earth. Low Earth orbit (LEO) is defined as orbital altitudes
below 2,000 km above the earth's surface and is the subject of the
image of space debris around Earth. Middle Earth orbit (MEO) is the
province of the Global Positioning System (GPS) and Russian navigation
satellite systems and is located at approximately 20,000-km altitude,
whereas the geosynchronous Earth orbit (GEO) "belt" is
inhabited primarily by communications and Earth—observation
payloads around 35,800 km. The majority of objects in these orbital
regions are in circular or near-circular orbits about the earth. In
contrast, the elliptical orbit category includes rocket bodies left in
their transfer (payload delivery) orbits to MEO and GEO as well as
scientific, communications, and Earth-observation payloads. Of all objects
listed in the

fragmentation debris table, the vast majority are
"debris"—only about 5 percent of objects in orbit
represent operational payloads or spacecraft. Also, of the approximately
28,000 objects that have been tracked, beginning with the launch of
Sputnik 1
in October 1957, those not accounted for in the fragmentation debris
table have either reentered the earth's atmosphere or have escaped
the earth's influence (to land on Mars, for example). The
distribution of debris smaller than 10 cm is predicated on the orbits of
the parent objects and is assumed to be very similar to the distributions
presented in the image of space debris around Earth.

Remediation Strategies

Remediation takes two courses: protection and mitigation. Protection seeks
to shield spacecraft and utilize intelligent design practices to minimize
the effects
of debris impact. Mitigation attempts to prevent debris from being
created. Active mitigation techniques include collision avoidance between
tracked and maneuverable objects and the intentional reentry of objects
over the oceans. Passive techniques include venting residual fuels or
pressurized vessels aboard rockets and spacecraft, retaining operational
debris, and placing spacecraft into disposal orbits at the end of a
mission. Space salvage or retrieval, while an option, is currently too
expensive to employ on a regular basis.

The United States and international space agencies recognize the threat of
debris and are cooperating to limit its environmental and health hazards.
The Interagency Space Debris Coordination Committee (IADC), sponsored
originally by the National Aeronautics and Space Administration (NASA),
has grown to include all major space-faring nations. The IADC charter
includes the coordination and dissemination of remediation research, and
strategies based on research results are being adopted by the worldwide
space community.

Remediation strategies have resulted in a decline in the rate of debris
growth in the 1990s although the overall population continues to grow.
Continued work is necessary, however, to reduce the orbital debris hazard
for future generations and continue the safe, economical utilization of
space.

Bibliography

Committee on Space Debris, Aeronautics and Space Engineering Board,
Commission on Engineering and Technical Systems, National Research
Council. (1995). "Orbital Debris: A Technical Assessment."
Washington, D.C.: National Academy Press. Also available from
http://pompeii.nap.edu/books/0309051258/html/index.html